Many computer displays have a limited physical resolution--typically 70-100 dots per inch (2.76 to 3.94 dots per mm). Since the display is composed of an array of rectangular pixels, each of which is either ON or OFF, the edges of text, lines, etc that are displayed may often have a jagged "staircase" effect which is visually disturbing.
There have been several attempts to solve this problem in a variety of ways e.g. by grey scale rendering or anti-aliasing. Some display technologies such as Twisted Nematic Liquid Crystal Displays (TN LCDs), cathode ray tubes, etc.,allow the intensity of the whole area of a pixel to be varied. In these types of display, the intensity of each pixel may be selected in proportion to the area of the pixel that should be ON. Whilst this can reduce the visually disturbing staircase effect, it can make edges in the displayed image appear blurred, especially when viewed from close to the display.
Other display technologies do not allow a range of intensities over the whole area of the pixel display, in which case a range of intensities may be simulated by rapidly turning the pixel ON and OFF, sufficiently fast so that the eye sees the average intensity. This may be used with Super Twisted Nematic (STN) LCDs. Alternatively, each pixel may be divided into sub pixels, and a varying number turned on according to the desired intensity. The display must then be viewed from such a distance that the eye cannot resolve the sub pixels, or some optical blurring introduced to average out the intensity over the whole pixel. An example of this type of technique is described in JP-A-3142260, where each pixel is effectively divided into four sub pixel slices. The image to be displayed is analysed and two-bit pixel data is added to each pixel to turn on selected sub pixel slices during a pixel sub-scanning period. This allows a range of intensities to be displayed by varying the area of the pixel that is ON, in four discrete steps. However, by its nature this system is only capable of modulating the pixel output slice-wise and in many instances this will not give good smoothing, particularly where the edge to be smoothed is nearly perpendicular to the slice direction of the pixels.
A similar technique is disclosed in U.S. Pat. No. 4,824,218, which relates principally to Ferroelectric LCDs. Here a variable width portion of a pixel is turned on by driving a potential gradient across the width of the pixel by means of metal electrodes running along the edges of resistive transparent column electrodes. To allow the complete area of the pixel to be driven, whilst preventing crosstalk (i.e. unintentionally affecting other pixels in the same row), and to avoid a wasted area of the pixel nearest the "reference" metal electrode, the pixel is driven in two phases, swapping the role of the two metal electrodes between "reference" and "data". This technique relies on the fact that a Ferroelectric Liquid Crystal (FLC) material stores its state and can be written to again, adding to the area that has already been turned ON, which is not true for all bistable materials. This scheme also requires a blanking pulse to clear the whole pixel before the two writing phases. U.S. Pat. No. 4,824,218 also refers to an extension of this technique in which the display has row and column transparent resistive electrodes each with metal electrodes on either side. A four field drive scheme is described in which the display is scanned four times, following a blanking pulse, to make up a frame. As before the metal electrodes swap roles between data and reference electrodes. Because alternate electrodes are set to a fixed reference this arrangement does not allow great flexibility in the creation of sub pixel shapes.